[ALSA] AD1888 mixer controls for DC mode

This patch adds two mixer controls. The V_REFOUT enable is a
documented register that couples the microphone input lines
to the V_REFOUT DC source. The High Pass Filter enable in the
AC97_AD_TEST2 (0x5c) is an undocumented register provided by
Miller Puckette via Analog Devices that enables the AD codec
to apply a high pass filter to the input.

Signed-off-by: Jaya Kumar <jayakumar.alsa@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>
diff --git a/include/sound/ac97_codec.h b/include/sound/ac97_codec.h
index 446afc3..758f8bf 100644
--- a/include/sound/ac97_codec.h
+++ b/include/sound/ac97_codec.h
@@ -265,6 +265,7 @@
 
 /* specific - Analog Devices */
 #define AC97_AD_TEST		0x5a	/* test register */
+#define AC97_AD_TEST2		0x5c	/* undocumented test register 2 */
 #define AC97_AD_CODEC_CFG	0x70	/* codec configuration */
 #define AC97_AD_JACK_SPDIF	0x72	/* Jack Sense & S/PDIF */
 #define AC97_AD_SERIAL_CFG	0x74	/* Serial Configuration */